Typically, current microphone assemblies for telecommunication applications are constituted by two terminal ECMs. A two terminal ECM assembly typically comprises a housing, an electret condenser microphone cartridge, and an integrated preamplifier, the assembly having only two terminals outside the housing: one terminal for ground and one terminal for signal output combined with supply voltage. The advantage of the two terminal design within cost sensitive telecommunication equipment is that the production costs can be decreased with reduced (e.g. from three to two terminals) number of terminals. Microphone assemblies for other applications, for example hearing aids, typically have three terminals: ground, output, and supply voltage. Within hearing aids other parameters control the preferred designs.
Currently, the preamplifiers used in such two terminal ECMs are designed using a single JFET transistor in a gain or a unity gain configuration. This has been a successful solution for many years providing low noise and gain in two terminal telecommunication ECM assemblies. But recently, the size of the ECM cartridges has shrinked dramatically, whereby the intrinsic sensitivity of the ECM cartridge is decreased compared to todays standards. Also the source capacitance of the ECM cartridge itself has been dramatically reduced—from the order of 10 pF down to typically 2-2.5 pF.
Prior art, constituted by a preamplifier with a JFET, typically has an input capacitance of 5-10 pF thus making the input capacitance of the preamplifier high compared to the ouput capacitance of the microphone cartridge. Therefore, the signal from the cartridge is unintentionally damped already before the first amplifier stage, thus lowering the sensitivity of the entire microphone assembly. Combined with the low sensitivity of the ECM cartridges themselves, the result is a poor noise performance e.g. expressed as a low Signal-to-Noise-Ratio (SNR).
A possible solution to the SNR problem is to use a MOS transistor in a source follower configuration as input device instead of a JFET. This may provide a very low input capacitance as well as low noise. However, using a MOS transistor as input device will give problems if gain is needed—such as normally required for use in a two terminal microphone assembly for telecommunication equipment. In order to achieve gain in a two terminal ECM traditionally a NMOS transistor in a ‘common source’ configuration is needed. If this should exhibit low noise then the NMOS transistor needs to be fairly large in size. On the other hand a large NMOS will have a fairly large input capacitance. A PMOS transistor may then in principle be used. However, in practice it has a number of disadvantages such as a large input capacitance and a poor power supply rejection ratio (PSRR) as it will reference the positive supply rail.
EP 0 969 695 A1 describes a preamplifier for use within a different technical field, hearing aids. The preamplifier described in EP 0 969 695 A1 enables an input capacitance of only 1.6 pF (column 4, line 46). However, the design has a number of disadvantages regarding use within telecommunication equipment, even though it is stated in column 3, lines 3-5, that the invention is applicable within this field as well. Within hearing aids it is not necessary to amplify the signal in the preamplifier stage due to the use of more expensive but high sensitivity microphone cartridges, whereas there is a requirement for an amplification factor larger than 1 within telecommunication equipment featuring low cost and low sensitivity cartridges. Therefore, preamplifiers for use within hearing aids are optimised with respect to low input capacitance only, and not with respect to large amplification. In addition, the preamplifier disclosed in EP 0 969 695 A1 requires three terminals out from the microphone assembly housing, namely the terminals marked “−”, “+”, and “OUT” in FIGS. 3, 4a and 4b. For applications within telecommunication equipment it is a strict requirement to have only two terminals from the microphone assembly. Therefore, the amplifier design described in EP 0 969 695 is not suitable for use within telecommunication equipment.
In U.S. Pat. No. 6,160,450, a two terminal microphone preamplifier is disclosed. This amplifier uses the priniciple of a differential input stage consisting of two PMOS transistors and a ‘open collector’ bipolar transistor as output device. Major disadvantages of the design of U.S. Pat. No. 6,160,450 are the introdution of additional noise from using a differential pair at the input, the Miller multiplication of gate-drain capacitance in the input stage adding to overall input capacitance of the preamplifier and finally the use of a mixed MOS/bipolar technology which may increase costs to realize the circuit.
In conclusion, the commonly known preamplifier designs for two terminal ECM miniature microphone assemblies known within the art of telecommunication equipment are insufficient to comply with the demands arising from the latest miniature ECM cartridges. Even though solutions to some of the new problems can be found within related arts, such as hearing aids, these solutions give rise to other problems that make them unsuitable within telecommunication equipment. These technical problems relate to the number of terminals required and the provided gain.